Logs are normally transported from a logging site to a sawmill or other destination by a relatively large truck-and-trailer combination. The truck includes a rearwardly extending bed equipped with an upwardly projecting, substantially U-shaped frame known as a bunk. The trailer is disengageably connectable to the truck by a relatively long, telescopic reach and also includes an upwardly extending U-shaped bunk. Axle-and-wheel assemblies, provided on both the truck and trailer, support the combination and distribute its weight over the roadway.
When a load of logs is to be hauled, the reach is first extended to provide a desired spacing between the truck and trailer bunks. The truck and trailer hitches are then appropriately coupled and logs are loaded and secured to the truck and trailer bunks. At this time, the logs are ready to be transported to, for example, a sawmill and their weight is relatively uniformly dispersed over the roadway. At the sawmill, the logs are unloaded and the truck-and-trailer combination is made ready to return to the logging site for another load of logs.
As will be appreciated, the trailer serves no load-carrying function during a return to the logging site. Moreover, the trailer is an impediment to maneuverability, particularly given the typically narrow, winding nature of logging roads. For that reason, prior art truck-and-trailer combinations have provided for the placement of the trailer on top of the truck bed prior to the combination being returned to the logging site. As a result, the maneuverability of the combination is significantly improved for the return trip. When this "piggyback" combination reaches the logging site, the trailer is hoisted from the truck bed by conventional logging equipment and is reconnected to the truck so that another load of logs can be hauled.
Given the extreme weight of the loads to be hauled by such truck-and-trailer combinations, it is frequently desirable to distribute the load over as many axle-and-wheel assemblies as possible. With multiple fixed axle-and-wheel assemblies provided on a single trailer, however, the tracking ability of the trailer is decreased significantly. More particularly, because such axle-and-wheel assemblies do not parallel the front wheels of the truck during a turn, they introduce a drag resistance that must be overcome by the truck. Drag resistance increases in proportion to the number of axle-and-wheel assemblies included on the trailer and, at some point, becomes intolerably large. The problem of reduced tracking ability is particularly acute for trailer systems that are required to negotiate the narrow, sharp curves frequently encountered on logging roads.
As an alternative, auxiliary or secondary trailers have been included to distribute the weight of the load across additional axle-and-wheel assemblies. Because the trailers are pivotally connected, the additional axle-and-wheel assemblies offer less drag resistance during a turn than they would if coupled directly to the first trailer.
While double-trailer configurations are relatively widely used, they are not without disadvantage. For example, the addition of an auxiliary trailer decreases maneuverability significantly when the truck-and-trailer is driven backwards. Further, in applications where the empty trailer is to be positioned upon the truck bed for transportation to a loading site, the addition of another trailer may result in an unwieldy and uncontrollable assembly that is difficult to load and that may further result in equipment damage or personal injury.
Another problem frequently encountered in the transportation of loads with trailers is presented by the existence of terrain variations at the loading or unloading site. More particularly, if the truck and trailer are loaded on uneven ground, it may be difficult to couple or uncouple the hitches. In some circumstances, the terrain variations may be so pronounced that the equipment must either be moved to level ground or a leveling adjustment performed.
In light of the foregoing observations, it would be desirable to provide a trailer system that offers enhanced load-carrying capacity, without adversely affecting the maneuverability of the truck-and-trailer combination. In addition, it would be helpful to provide a trailer system that can be easily loaded and unloaded from a truck bed when empty. Finally, it would be advantageous for the truck and trailer hitches to be easily coupleable independent of terrain variations below the truck and trailer.